# 1:1:4 Wind Speed

Wind speed is required by SWAT+ if the Penman-Monteith equation is used to estimate potential evapotranspiration and transpiration. SWAT+ assumes wind speed information is collected from gages positioned 1.7 meters above the ground surface. 

When using the Penman-Monteith equation to estimate transpiration, the wind measurement used in the equation must be above the canopy. In SWAT+, a minimum difference of 1 meter is specified for canopy height and wind speed measurements. When the canopy height exceeds 1 meter, the original wind measurements is adjusted to:

$$z\_w=h\_c+100$$                                                                                                                                                       1:1.4.1

where $$z\_w$$ is the height of the wind speed measurement (cm), and $$h\_c$$ is the canopy height (cm). 

The variation of wind speed with elevation near the ground surface is estimated with the equation (Haltiner and Martin, 1957):     

$$u\_{z2}=u\_{z1}\[\frac{z\_2}{z\_1}]^{aa}$$                                                                                                                                                      1:1.4.2

where $$u\_{z1}$$ is the wind speed (m s$$^{-1}$$) at height $$z\_1$$ (cm), $$u\_{z2}$$ is the wind speed (m s$$^{-1}$$) at height $$z\_2$$ (cm), and $$aa$$ is an exponent between 0 and 1 that varies with atmospheric stability and surface roughness. Jensen (1974) recommended a value of 0.2 for $$aa$$ and this is the value used in SWAT+. 

The daily wind speed data required by SWAT+ may be read from an input file or generated by the model. The variable wnd in the master weather ([weather-sta.cli](https://swatplus.gitbook.io/io-docs/introduction-1/climate/weather-sta.cli)) file identifies if there is available input  wind speed data or if it will be simulated.  The [wnd.cli](https://swatplus.gitbook.io/io-docs/introduction-1/climate/wnd.cli-and-wind-speed-data-files) file includes a list of all available wind speed data stations, and can be empty if all locations are simulated. To read in daily wind speed data, the variable is set to  the name of the wind speed data station. To generate daily wind speed values wnd is set to "sim". The equations used to generate wind speed data in SWAT+ are reviewed in Chapter 1:3.

Table 1:1-9: SWAT+ input variables used in wind speed calculations.

| Definition                                                                                  | Source Name   | Input Name | Input File                                                                                              |
| ------------------------------------------------------------------------------------------- | ------------- | ---------- | ------------------------------------------------------------------------------------------------------- |
| List of measured wind speed station names \[input IDs]                                      | wnd\_filename | wnd\_file  | [wnd.cli](https://swatplus.gitbook.io/io-docs/introduction-1/climate/wnd.cli-and-wind-speed-data-files) |
| Measured wind speed station name \[input ID] (##.wnd) (to simulate wind speed set to "sim") | wnd\_filename | wnd        | [weather-sta.cli](https://swatplus.gitbook.io/io-docs/introduction-1/climate/weather-sta.cli)           |

See description of .wnd file on the [wnd.cli](https://swatplus.gitbook.io/io-docs/introduction-1/climate/wnd.cli-and-wind-speed-data-files) page for input and format requirements if measured daily wind speed data is being used.